Hello I will answer your question,
i have two of these as daily drivers and the first one had the same issues as yours. The oil rings will get sludged and carboned to the point they will no longer do their job of scraping the oil from the cylinder walls. The oil is burned in the combustion chamber and the catalytic converter so you do not see any smoke out the tailpipe. I think you will find the throttlebody gets very black quickly. This is due to poor sealing of the valves and the exhaust reverting up the intake tract when the valves have so much carbon on them they do not seat properly. The sparkplugs will also show carbon deposits that eventuall leads to plug fouling.
If this is a SOHC there is an inherent weakness of the cylinder head between the #3 and #4 cylinders where they crack and leak that was adressed in later builds.
With all that said there are thousands of these on the road and many of them have these same problems yet they keep getting their owners where they need to go everyday.
Of course the only true fix is to overhaul or replace the engine. I found a complete wrecked vehicle that had sat for six years and swapped the engine then rebuilt the original and sold that for the price of the wreck. That is one way to go.
I hope you find this answer helpful
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